Overflow chamber

ABSTRACT

An overflow chamber ( 2 ) is provided for use in a combined sewer overflow. An inlet ( 12 ) and an outlet ( 14 ) open into the chamber ( 2 ) below a screen ( 20 ), and an overflow outlet ( 15 ) opens into the chamber above the screen. Baffles ( 26 ) are provided underneath the screen ( 20 ) across the general flow of storm water. In storm conditions, the storm water in the chamber ( 2 ) will reach the level of the screen ( 20 ), which traps aesthetically offensive material and gross solids, leaving the water to flow through the screen ( 20 ) to the overflow outlet ( 15 ). The baffles ( 26 ) circulate the storm water, and this dislodges material accumulated on the underside of the screen ( 20 ) and so helps to prevent blinding of the screen ( 20 ) in storm conditions.

This invention relates to an overflow chamber and is particularly,although not exclusively, concerned with an overflow chamber for use ina combined sewer overflow (CSO). The present invention is also concernedwith a screen for use in such an overflow chamber.

CSOs are known which comprise a chamber divided horizontally by ascreen. An inlet and an outlet open into the chamber below the screen,and an overflow outlet opens into the chamber above the screen. Theinlet is of larger diameter than the outlet. In normal operation, sewageand rain water flow into the chamber through the inlet at a relativelylow flow rate, and pass through the outlet to the sewage system.

In storm conditions, the flow of storm water through the inlet isgreater than the outlet will accept. Consequently, water and sewagebuilds up within the chamber. Eventually, the storm water in the chamberwill reach the level of the screen, which traps aesthetically offensivematerial and gross solids, leaving the water to flow through the screento the overflow outlet. As the storm recedes, and the water level in thechamber drops below the screen, solids accumulated on the screen fallaway and pass with the remaining water to the sewage system through theoutlet.

A problem with such screens is that solids may accumulate on the screenunder storm conditions, so that it is wholly or partially “blinded” andthe ability of water to flow through the screen is reduced. Under suchcircumstances, water, with entrained gross solids, may flow through anemergency bypass to a watercourse.

According to the present invention there is provided an overflow chambercomprising an inlet and an outlet, a screen disposed above the inlet andoutlet whereby flow may take place between the inlet and the outletwithout passing through the screen, and an overflow outlet disposedabove the screen, at least one baffle being provided in the chamberbelow the screen, the baffle extending across the general flow directionbetween the inlet and the outlet.

The effect of the baffle is to create, within the chamber below thescreen, a series of compartments within which storm water circulates insuch a manner as to sweep the underside of the screen, thereby todislodge material accumulated on the screen.

There may be at least two of the baffles, and they may be fixed to thescreen and project downwardly from it. They preferably stop short of thefloor of the chamber, and may, for example, have a vertical extent whichis approximately three times the vertical depth of the screen.

The inlet and the outlet may open into the chamber opposite each other,and the flow cross-section of the outlet is preferably smaller than theflow cross-section of the inlet.

For a better understanding of the present invention and to show how itmay be carried into effect reference will now be made, by way ofexample, to the accompanying FIGURE, which shows a combined seweroverflow.

The overflow shown in the FIGURE comprises a chamber 2 of generallyrectangular shape, as seen in plan view, comprising end walls 4 and 6and side walls 8 and 10. An inlet 12 is provided in the end wall 4, andan outlet 14 is provided in the end wall 6. The inlet 12 and the outlet14 are generally opposite one another, although the outlet 14 is at aslightly lower level than the inlet 12. The floor 16 of the chamber 2has a gully 18 which channels sewage under low-flow conditions betweenthe inlet 12 and the outlet 14.

A screen 20 is provided in the chamber 2. It is generally horizontal,and may, for example, comprise a corrugated apertured panel which may beprovided with a non-stick plastic coating. By way of example, theapertures in the screen may have a diameter (or equivalent transversedimension) of 4 mm.

As shown in the FIGURE, the screen 20 has a frame 22 which is secured toat least some of the walls 4, 6, 8, 10 of the chamber 2. The screen 20stops short of the end wall 4, and is provided at that end with a weirplate 24. Baffles 26 are provided on the underside of the screen 20, andextend substantially the full width of the screen 20. The baffles 26 aredisposed vertically, and stop short of the floor 16 of the chamber 2.Two of the baffles 26 are provided, disposed at approximately equalintervals along the length of the screen 20. It will be appreciated thatthe baffles 26 divide the region of the chamber 2 beneath the screen 20into three compartments 28 of substantially equal length, whichcompartments are intercommunicate with one another through the gaps leftbetween the lower edges of the baffles 26 and the floor 16.

The chamber is installed in a sewage system, so that the inlet 12 isconnected to receive sewage and rain water, and the outlet is connectedto downstream components of the sewage system. An overflow outlet 15 isconnected, for example, for discharge into a river or other watercourse.

Under normal flow conditions, sewage and other water, for example lightrainfall, entering the system will pass through the inlet 12 and flowalong the gully 18 to the outlet 14. Under heavier flow, for exampleunder storm conditions, the smaller flow cross-section of the outlet 14will not be able to accept the full incoming flow through the inlet 12,and consequently the water level in the chamber 2 will rise. As itrises, it carries with it entrained aesthetically offensive material andgross solids. Eventually, the level will reach the screen 20, which willhave the effect of intercepting the solids material allowing only thewater to reach the overflow outlet 15 and the watercourse into which itdischarges.

Under these heavy flow conditions, the flow pattern within the chamber 2below the screen 20 is modified by the baffles 26. Thus, as seen invertical cross-section, flow striking each baffle (and the end wall 6)will be deflected upwards and will undergo a swirling motion about ahorizontal axis extending transversely across the chamber between thewalls 8 and 10. Consequently, a reverse flow (with respect to the normaldirection of flow between the inlet 12 and the outlet 14) will occuracross the underside of the screen 20, re-entraining much of thematerial deposited on the screen 20. Consequently, any blockage of theapertures in the screen 20 is minimised, so that flow through the screen20 can take place even when the flow entering through the inlet 12 isheavily contaminated by solids material. Nevertheless, shouldexceedingly high flow rates occur, and, despite the flushing action ofthe circulating flow in the compartments 28, the screen becomessubstantially blinded, the weir plate 24 provides an emergency overflowallowing full flow through the inlet 12 to overflow the weir plate 24and reach the overflow outlet 15 without passing through the screen 20.

As the storm subsides, the inflow through the inlet 12 will reduce,allowing the level in the chamber 2 to fall as the storm water andentrained solids material leave through the outlet 14. As a result ofthe circulating motion within the compartments 28 and the resultingreduction in settling of solids on the underside of the screen 20, mostof the solids materials within the chamber 2 will be able to leavethrough the outlet 14, leaving the screen 20 relatively unobstructed andable to operate effectively under subsequent storm conditions.

In the FIGURE, the baffles 26 are shown as flat, rectangular plates,although differently shaped plates may be suitable for enhancing theswirl effect generated by them. Also, it will be appreciated that thenumber and dimensions of the plates 26 may be varied to suit theconditions prevailing in any particular CSO.

The invention therefore provides effective screening in storm conditionsto intercept and retain aesthetically offensive material and grosssolids so that they do not reach watercourses under overflow conditions.The baffles 26 enhance flow circulation patterns within the chamber 2 toreduce the blinding effect of entrained solids when the screen is inoperation.

It will be appreciated that, although the invention has been describedwith reference to a combined sewer overflow, it may be applied toscreens used in other applications.

1. An overflow chamber comprising an inlet and an outlet, a screendisposed above the inlet and outlet, whereby flow may take place betweenthe inlet and the outlet without passing through the screen, and anoverflow outlet disposed above the screen, at least one baffle beingprovided in the chamber below the screen, the baffle extending acrossthe general flow direction between the inlet and the outlet.
 2. Anoverflow chamber as claimed in claim 1, wherein the baffle is one of atleast two baffles.
 3. An overflow chamber as claimed in claim 1, whereinthe or each baffle is fixed to the screen and projects downwardly fromthe screen.
 4. An overflow chamber as claimed in claim 1, wherein the oreach baffle stops short of the floor of the chamber.
 5. An overflowchamber as claimed in claim 4, wherein the or each baffle has a verticalextent which is approximately three times the vertical depth of thescreen.
 6. An overflow chamber as claimed in claim 1, wherein the oreach baffle comprises a flat rectangular plate.
 7. An overflow chamberas claimed in claim 1, wherein the inlet and the outlet are disposed onopposite sides of the chamber.
 8. An overflow chamber as claimed inclaim 1, wherein the flow cross-section of the outlet is smaller thanthe flow cross-section of the inlet.